The present invention relates generally to data transmission in mobile communication systems and more specifically to a channel state information (CSI) reference signal (RS) to support coordinated multi-point network implementations and heterogeneous networks.
As used herein, the terms “user equipment” and “UE” can refer to wireless devices such as mobile telephones, personal digital assistants (PDAs), handheld or laptop computers, and similar devices or other User Agents (“UAs”) that have telecommunications capabilities. A UE may refer to a mobile, or wireless device. The term “UE” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems/equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” or “access device” will refer to any component, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UE with access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, a base station provides radio access to one or more UEs. The base station comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UEs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UEs, deciding the transport channel to be used for each UE's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UEs through a scheduling channel.
It is generally desirable to provide a high data rate coverage using signals that have a high Signal to Interference Plus Noise ratio (SINR) for UEs serviced by a base station. Typically, only those UEs that are physically close to a base station can operate with a very high data rate. Also, to provide high data rate coverage over a large geographical area at a satisfactory SINR, a large number of base stations are generally required. As the cost of implementing such a system can be prohibitive, research is being conducted on alternative techniques to provide wide area, high data rate service.
Coordinated multi-point (CoMP) transmission and reception may be used to increase transmission data rate and/or signal quality in wireless communication networks such as LTE-A networks. Using CoMP, neighboring base stations coordinate to improve the user throughput or signal quality, especially for users at a cell edge. CoMP may be implemented using a combination of base stations such as eNBs, and/or relay nodes (RN) and/or other types of network nodes and/or cells.
FIG. 1 is an illustration of a wireless communications network having two eNBs operating in a CoMP transmission and reception configuration. A similar illustration can be applied to a combination of eNBs, RNs and/or cells. As illustrated in FIG. 1, in network coverage area 104, eNBs 106 and 108 are configured to transmit communication signals to UE 110. In network coverage area 104, any collaboration scheme may be used for eNBs 106 and 108. For example, in some CoMP schemes, eNB 106 and eNB 108 may work together to transmit the same signal to UE 110 at the same time. In such a system, the signals transmitted by the base stations combine (i.e., superpose) in the air to provide a stronger signal and thus increase the chance of transmission success. In other CoMP schemes, eNB 106 and eNB 108 transmit different signals to UE 110, which, for example, include different data that is to be communicated to UE 110. By transmitting different portions of the data through different eNBs, the throughput to UE 110 may be increased. The use of CoMP depends on many factors including channel conditions at UE 110, available resources, Quality of Service (QoS) requirements, etc. As such, in some network implementations, in a given node/cell or combination of nodes/cells only a subset of available UEs may be serviced with CoMP transmissions. For example, in FIG. 1, UE 112 is only served by eNB 108.
In LTE-A, CoMP can be used to improve the throughput for cell edge UEs as well as the cell average throughput. There are two primary mechanisms by which CoMP transmissions may be implemented to recognize these improvements. First, CoMP transmissions may provide coordinated scheduling, where data is transmitted to a single UE from one of the available transmission points (e.g., one of the available eNBs in FIG. 1 or one of the available network nodes or cells) and scheduling decisions are coordinated to control, for example, the interference generated in a set of coordinated cells. Secondly, CoMP transmissions may provide joint processing where data is simultaneously transmitted to a single UE from multiple transmission points, for example, to (coherently or non-coherently) improve the received signal quality and/or actively cancel interference for other UEs.
In the case of coordinated scheduling, data is only transmitted by the serving cell, but the scheduling decisions are made with coordination among the neighboring cells. In the case of joint processing CoMP transmission, multiple base stations transmit the data to the same user simultaneously. The UE then jointly processes the transmissions from multiple nodes to achieve a performance gain.
In CoMP implementations, the serving cell may be the cell transmitting Physical Downlink Control Channel (PDCCH) assignments (i.e., a single cell). This is analogous to the serving cell of Rel-8. In CoMP, dynamic cell selection involves a PDSCH transmission from one point within the CoMP cooperating set at a first time and in Coordinated Scheduling/Beamforming (CS/CB) data is only available at the serving cell (data transmission from that point) but user scheduling/beamforming decisions are made with coordination among cells corresponding to the CoMP cooperating set.
When implementing CoMP, a series of CoMP cell sets may be defined. In a CoMP cooperating set, a set of (geographically separated) points directly or indirectly participate in PDSCH transmission to the UE. The cooperating set may be transparent to the UE. CoMP transmission point(s) are a point or set of points actively transmitting PDSCH to the UE. CoMP transmission point(s) are a subset of the CoMP cooperating set. For joint transmission, the CoMP transmission points are the points in the CoMP cooperating set, but for dynamic cell selection, a single point is the transmission point at each subframe. The transmission point can change dynamically within the CoMP cooperating set. A CoMP measurement set is a set of cells about which channel state/statistical information (CSI) related to their link to the UE is reported. The CoMP measurement set may be the same as the CoMP cooperating set. A Radio Resource Measurement (RRM) measurement set is a set in support of RRM measurements that may be defined in Rel-8 and is, therefore, not CoMP-specific. For Coordinated scheduling/beamforming, the CoMP transmission point may correspond to the “serving cell.”
In LTE systems, data is transmitted from an access device to UEs via Resource Blocks (RBs). Referring to FIG. 2, an exemplary resource block 50 is illustrated that is comprised of 168 Resource Elements (REs) (see exemplary elements 52) arranged in twelve frequency columns and fourteen time rows as known in the art. Accordingly, each element corresponds to a different time/frequency combination. The combination of elements in each time row are referred to as an Orthogonal Frequency Division Multiplexing (OFDM) symbol. In the illustrated example the first three OFDM symbols (in some cases it may be the first two, first four, etc.) are reserved for PDCCH 56 and are shown in FIG. 2 as gray REs collectively. Various types of data can be communicated in each RE.
LTE systems employ various types of reference signals to facilitate communication between an access device or base station and a UE. A reference signal can be used for several purposes including determining which of several different communication modes should be used to communicate with UEs, channel estimation, coherent demodulation, channel quality measurement, signal strength measurements, etc. Reference signals are generated based on data known to both an access device and a UE, and may also be referred to as pilot, preamble, training signals, or sounding signals. Exemplary reference signals include a cell specific reference signal (CRS) that is sent by a base station to UEs within a cell and is used for channel estimation and channel quality measurement, a UE-specific or dedicated reference signal (DRS) that is sent by a base station to a specific UE within a cell that is used for demodulation of a downlink, a sounding reference signal (SRS) sent by a UE that is used by a base station for channel estimation and channel quality measurement and a demodulation reference signal sent (DM-RS) by a UE that is used by a base station for channel estimation of an uplink transmission from the UE.
In LTE systems, CRSs and DRSs are transmitted by base stations in RB REs. To this end, see FIG. 2 which shows an exemplary CRS (three of which are labeled 52) in vertical, horizontal, left down to right and left up to right hatching for ports 0 through 3 respectively and exemplary DRS in dark REs to the right of the three columns of PDCCH 56, three of which are labeled 54. The reference signals allow any UEs communicating with the access device to determine channel characteristics and to attempt to compensate for poor characteristics. The CRSs are UE-independent (i.e., are not specifically encoded for particular UEs) and, in at least some cases, are included in all RBs. By comparing the received CRS to known reference signals (i.e., known data), a UE can determine channel characteristics (e.g., a channel quality information, etc.). The difference between the known data and the received signal may be indicative of signal attenuation, path-loss differences, etc.
UEs report channel characteristics back to the base station and the base station then modifies its output (i.e., subsequent REs) to compensate for the channel characteristics. To indicate how signal output is modified, the base station transmits a UE-specific DRS to each UE. Here again, DRS data is known at the UE and therefore, by analyzing received DRS the UE can determine how the access device output has been modified and hence obtain information required to demodulate data received in subsequent REs. In FIG. 2, exemplary CRS reference signals are indicated by hatching, DRS signals are indicated by dark REs and non-reference signal elements during which traffic data is transmitted are blank (i.e., white).
Referring again to FIG. 2, to avoid collisions, LTE system DRS 54 are generally allocated to OFDM symbols separate from those occupied by CRS. Furthermore, DRS 54 are generally allocated away from PDCCH 56. In release 8 LTE devices (hereinafter “Rel-8 devices”), for example, DRS of antenna port 5 may be specified for PDSCH demodulation as shown in FIG. 2. In some cases, CRS 52 on antenna ports 0-3 are distributed on all RBs in the system bandwidth, while DRS 54 on antenna port 5, for example, may only be allocated in RBs assigned to a corresponding UE. When a UE is assigned two or more contiguous RBs, DRS 54 allocation may simply be repeated from one RB 50 to the next.
Two new types of reference signals are defined in LTE-A for the purpose of channel estimation for demodulation: channel estimation for channel state information (CSI) measurement and channel quality indicator (CQI) measurement. The first type of RS is a UE-specific RS or UE-RS used for demodulation of the traffic channel assigned to the UE, i.e. the physical downlink shared channel (PDSCH). The UE-RS is also called demodulation RS (DM-RS). The second type of RS is a cell-specific RS used for CSI measurement and CQI measurement. In LTE-A, the LTE Rel-8 common reference signal (CRS) may be retained in non-Multicast/Broadcast over a Single Frequency Network (MBSFN) subframes to support legacy Rel-8 UEs. In an MBSFN subframe which may be used as a subframe to only support LTE-A UE, CRS may only be retained within the PDCCH region.
In some network implementations, then anticipated CSI-RS overhead is approximately 1/840=0.12% per antenna port (8 antenna ports=0.96%). For example, CSI-RS may be implemented with a time density of 1 symbol every 10 ms per antenna port: 1/140, or a frequency density of 1 subcarrier every 6 subcarriers per antenna port: ⅙. The periodicity of the CSI-RS signal may be adjusted by an integer number of timeframes. For DM-RS the broadcast rate is: Rank 1 transmission—12 REs per RB (same overhead as Rel-8); Rank 2 transmission—12 REs per RB to be confirmed, and Rank 3-8 transmissions—a maximum of 24 REs (total) per RB. Generally, the same REs per antenna port are transmitted for each DM-RS rank.
There are several difficulties associated with current CSI-RS designs. First, to support CoMP multi-cell CSI measurement at the UE, the UE is required to detect the CSI-RS transmitted by neighboring cells with a sufficient level of accuracy. However, because the signal strength received from neighboring cells can be relatively low compared to the signal strength received from the serving cell and the sum of the signal strength received from other neighboring cells, the received SINR of a neighboring cell CSI-RS can be quite low.
Also, existing CSI-RS design focuses on a homogeneous network scenario where only macro cells are deployed. Future networks, however, may be implemented using heterogeneous networks incorporating macro cells overlaid with small cells (also called low power nodes, e.g. femto cell, relay cell, pico cell etc.). In that case, the expected reuse cluster size will need to a much larger than the 6 to 8 cluster size currently specified. Because macro eNBs and small cell eNBs have very different transmit power (the transmit power of a macro eNB is 46 dBm (for 10 MHz bandwidth) whereas the transmit power of a pico eNB, femto eNB and relay node (RN) is 30 dBm, 20 dBm and 30 dBm respectively for 10 MHz bandwidth), the larger transmit power of the macro eNB will lead to severe DL interference experienced by a UE attached to the low power node that is located within the macro eNB coverage. This severe outer-cell interference will be detrimental to the performance of control channels (e.g. PDCCH), data channels (e.g. PDSCH) and RS detection, including CSI-RS detection.
Finally, to support CoMP with higher reuse cluster sizes and multi-antenna configurations, the number of CSI-RS antenna ports will be significant. To limit overhead, a larger periodicity of the CSI-RS may be required. A larger interval between CSI-RS transmissions may negatively affect detection performance of CSI-RS for a higher speed mobile that may, or may not, be in CoMP operation.